Dynamics of the Interaction of Formic Acid with a Polycrystalline Pt Film Electrode: a Time-Resolved ATR-FTIR Spectroscopy Study at Low Potentials and Temperatures

Aiming at more insights into the interaction of formic acid with a Pt electrode, we have studied the dynamics of formic acid interaction with a polycrystalline Pt film electrode in the potential range around the onset of the reaction, from 0.0 to 0.4 V (reversible hydrogen electrode (RHE)), by a combination of electrochemical and in situ IR spectroscopy transient measurements. The measurements were performed under well-defined mass transport conditions; IR spectra were acquired in an attenuated total reflection (ATR) configuration with a time resolution of up to 25 ms (rapid scan mode). To slow down the reaction kinetics and thus stabilize short-living adsorbed intermediates, measurements were performed at ambient and low reaction temperatures (3 degrees C). Kinetic H/D isotope effects, introduced by using deuterated formic acid, were explored to learn more about the contribution of C-H bond splitting in the rate-determining step in formic acid dehydration (COad formation). Rapid scan ATR-FTIRS measurements show no measurable time delay between the appearance of the bands related to adsorbed bridge-bonded formate species and adsorbed CO at higher potentials (> 0.1 V) and no detectable formate signals at low potentials (<= 0.1 V), although COad is still formed even at 0.0 V. Adsorption of HCOOH species at low potentials (0.0-0.1 V-RHE) is indicated by a band developing at around 1720 cm(-1), which is isotope shifted upon deuteration of the C-H bond. Consequences of these and other observations, including the observation of a bell-shaped potential dependence of the initial rate for COad formation in the potential range 0.0-0.4 V or distinct kinetic H/D isotope effects in the rate constants for COad formation, on the mechanistic understanding of the formic acid-Pt interaction are discussed.